1. Field of the Invention
The present invention relates to a hydraulic circuit of an option device for an excavator which can operate an option device such as a breaker, a hammer, a shear, and so forth, mounted on an excavator.
More particularly, the present invention relates to a hydraulic circuit of an option device for an excavator, which can constantly supply hydraulic fluid fed from a hydraulic pump to the option device irrespective of the size of load occurring when the option device operates, and can control respective flow rates required for various kinds of option devices.
2. Description of the Prior Art
As illustrated in FIGS. 1 and 2, a conventional hydraulic circuit of an option device for an excavator includes variable displacement hydraulic pump 26; an option device 24 (e.g., a breaker and so on) connected to the hydraulic pump 26; a first spool 15 installed in a flow path between the hydraulic pump 26 and the option device 24 and shifted to control hydraulic fluid being supplied to the option device 24 through an option port 22 in response to a pilot signal pressure Pi applied thereto; a poppet 14 installed in a flow path between the hydraulic pump 26 and the first spool 15 to control hydraulic fluid fed from the hydraulic pump 26 to the option device 24 when the first spool 15 is shifted; a piston 13 elastically supported in a back pressure chamber 17 of the poppet 14; and a second spool 3 shifted to control hydraulic fluid fed from the hydraulic pump 26 to the back pressure chamber 17 of the poppet 14 through a flow path 23 connected to the back pressure chamber 17, in response to a difference between a pressure of an inlet part of the first spool and a sum of a pressure of an outlet part of the first spool 15 and an elastic force of a valve spring 5.
The conventional hydraulic circuit of an option device for an excavator further includes a first orifice 13a formed in the piston 13 and controlling hydraulic fluid fed from the hydraulic pump 26 to the back pressure chamber 17 of the poppet 14 when the second spool 3 is shifted; a second orifice 30 formed in a flow path 23 between the second spool 3 and a back pressure chamber 29 of the piston 13, and controlling hydraulic fluid fed from the hydraulic pump 26 to the back pressure chamber 29 when the second spool 3 is shifted; and a third orifice 31 installed in a flow path 16 having an inlet part connected to a flow path between the first spool 15 and the poppet 14 and an outlet part connected to the second spool 3, and controlling hydraulic fluid which is fed from the hydraulic pump 26 to shift the second spool 3.
In the drawing, reference numeral 19 denotes a pilot flow path connected to a supply line 20 of the hydraulic pump 26 to receive a signal pressure for shifting the second spool 3.
Hereinafter, the operation of the conventional hydraulic circuit of an option device will be described.
As shown in FIGS. 1 and 2, the hydraulic fluid fed from the hydraulic pump 26 is supplied to the supply line 20 and the pilot flow path 19. The hydraulic fluid fed to the supply line 20 pushes the poppet 14 upward as shown in the drawing.
The hydraulic fluid fed to the back pressure chamber 17 of the poppet 14 is supplied to a chamber 21 through an orifice 14a of the poppet 14, and thus the poppet 14 is moved upward to be in contact with the piston 13 (in this case, the elastic member 12 is compressed). Accordingly, the hydraulic fluid on the supply line 20 is supplied to the chamber 21.
When the pilot signal pressure Pi is applied to a left port of the first spool 15, the first spool 15 is shifted in the right direction. The hydraulic fluid fed to the chamber 21 is supplied to the option device 24 through the option port 22 to drive the option device 24.
In this case, when the chamber 21 and the option port 22 are connected together by the shifting of the first spool 15 and the hydraulic fluid is supplied to the option device 24, a loss in pressure occurs between a pressure before the hydraulic fluid passes through the second spool 3 and a pressure after the hydraulic fluid passes through the second spool 3.
As illustrated in FIG. 1, the pressure, which is increased due to the shifting of the first spool 15, is supplied to a left end of the second spool 3 along the flow path 16 connected to the chamber 21. When the hydraulic fluid is supplied to the second spool 3 after passing through the third orifice 31 formed at an end part of the flow path 16, the second spool 3 is shifted in the right direction as shown in the drawing (FIG. 2 illustrates the second spool 3 that is shifted in the left direction). In this case, if it is assumed that the cross-sectional area of a diaphragm of the second spool is A1, a force that shifts the second spool 3 in the right direction is (A1×P1).
The pressure in the option port 22 is applied to a right end of the second spool 3 after passing through the pilot flow path 18. Accordingly, the second spool 3 is shifted in the left direction as shown in the drawing (FIG. 2 illustrates the second spool 3 that is shifted in the right direction). In this case, if it is assumed that the cross-sectional area of the diaphragm of the second spool is A2, a force that shifts the second spool 3 in the left direction is (A2×P2)+F1 (which corresponds to the elastic force of the valve spring 5).
That is, the condition that the second spool 3 is kept in its initial state (which corresponds to the state as illustrated in the drawing) is given as (A1×P1)<((A2×P2)+F1), and the condition that the second spool 3 is shifted in the right direction is given as (A1×P1)>((A2×P2)+F1).
In the case of shifting the second spool 3 in the right direction as shown in FIG. 1, the hydraulic fluid is supplied to a left end of the second spool 3 through the flow path 16, and the second spool 3 is shifted in the right direction. The hydraulic fluid fed to the pilot flow path 19 is supplied to the back pressure chamber 29 of the piston 13 after passing through the second spool 3, and a through flow path 23 in order, and thus the piston is moved downward as shown in the drawing. Simultaneously, the poppet 14 elastically installed by the elastic member 12 is moved downward.
The flow path between the supply line 20 and the chamber 21 is blocked by the poppet 14. AS the pressure in the flow path 16 is reduced, the second spool 3 is moved in the left direction as shown in FIG. 1. This corresponds to the state given as (A1×P1)<((A2×P2)+F1).
When the second spool 3 is shifted in the left direction as shown in the drawing, the supply of the pressure in the pilot flow path 19 to the through flow path 23 is intercepted. As the poppet 14 is moved upward as shown in the drawing, the hydraulic fluid fed from the hydraulic pump 26 is supplied to the second spool 3 via the chamber 21 and the flow path 16. This corresponds to the state given as (A1×P1)>((A2×P2)+F1). Accordingly, the second spool 3 is shifted in the right direction as shown in the drawing.
As illustrated in FIGS. 4A and 4B, a loss in pressure occurring between the signal pressures for shifting the second spool 3 becomes constant due to the repeated shifting of the second spool 3.
That is, it is known that the flow rate Q of the hydraulic fluid being supplied to the option device 24 is Q=(Cd×A×ΔP). Here, Q denotes the flow rate, Cd denotes a flow rate coefficient, A denotes an opening area of a spool (A=constant), and ΔP denotes a loss in pressure between P1 and P2 (ΔP=constant).
As described above, in the conventional hydraulic control valve structure of an option device, the hydraulic fluid fed from the hydraulic pump 26 can be constantly supplied to the option device 24 irrespective of the size of a load occurring in the option device 24.
By contrast, as shown in FIG. 3, the flow rate of the hydraulic fluid being supplied to the option device is overshot (indicated as “a” in the drawing) in an initial control period of the option device, and then is stabilized with the lapse of a predetermined time. This may cause an abnormal operation of the option device in the initial operation period of the option device to lower the stability of the option device.
In addition, option devices have different specifications depending on their manufacturers. Although the flow rate and pressure required for the option devices may differ, the flow rate of the hydraulic fluid being supplied to various kinds of option devices is not controlled, but the same flow rate is always applied thereto.
Accordingly, even an operator having wide experience in operation cannot efficiently manipulate the option devices to lower the workability.